Run tests in parallel

[qba73]

Hi @tufanbarisyildirim, this PR proposes changes to test files to allow parallel test execution.

Testing locally:

➜  gonginx git:(paralleltests) ✗ go test -race ./... -count=1
ok  	github.com/tufanbarisyildirim/gonginx	0.217s
?   	github.com/tufanbarisyildirim/gonginx/examples/adding-server	[no test files]
?   	github.com/tufanbarisyildirim/gonginx/examples/formatting	[no test files]
?   	github.com/tufanbarisyildirim/gonginx/examples/update-directive	[no test files]
ok  	github.com/tufanbarisyildirim/gonginx/parser	0.335s
ok  	github.com/tufanbarisyildirim/gonginx/parser/token	0.160s

Testing locally using make target:

➜  gonginx git:(paralleltests) make test
go test -race -cover /Users/jakub/projects/gonginx/parser/token
ok  	github.com/tufanbarisyildirim/gonginx/parser/token	(cached)	coverage: 100.0% of statements
go test -race -cover /Users/jakub/projects/gonginx/parser
ok  	github.com/tufanbarisyildirim/gonginx/parser	(cached)	coverage: 95.3% of statements
go test -race -cover /Users/jakub/projects/gonginx
ok  	github.com/tufanbarisyildirim/gonginx	(cached)	coverage: 65.7% of statements

[tufanbarisyildirim]

cool, merging, thanks!